Liquid droplet ejection techniques are well known in the field of ink-jet printers and metal spray devices. Ink-jet printers typically create a printed image by ejecting water-based inks or hot-melt waxes through an orifice at an ambient or relatively low temperature. A wide variety of materials is used to manufacture the ejection mechanisms for these printers because of the low temperature of the printing process. For example, piezoelectric crystals are used to convert an electrical signal into an acoustical signal to eject a low temperature droplet of ink. In another example, a bundle of fine wires is used to transmit an ultrasonic pulse to an ink meniscus to eject an ink droplet at a low temperature. At temperatures greater than 600.degree. Celsius, however, piezoelectric materials and bundles of fine wires decompose or cease to function.
Metal spray devices, such as the apparatus shown in U.K. Pat. No. 1,472,939 to Osprey, typically are manufactured of materials that can withstand the high temperatures associated with ejection of liquid metal droplets. The ejected droplets, however, follow erratic trajectories and cannot be used to build precision structures such as those described in U.S. Pat. No. 5,257,657 to Gore. Other devices, such as the apparatus shown in U.S. Pat. No. 5,171,360 to Orme, produce a stream of metal droplets at 200.degree. Celsius with a precisely controlled trajectory but cannot produce discrete droplets on demand.
Accordingly, a need exists for an ejection method and apparatus that eject a discrete droplet of liquid on demand at a high temperature along a predetermined trajectory without the disadvantages of the prior art.